Refrigerator Interface for Cryostat

ABSTRACT

A substantially cylindrical cryostat defining an axial imaging region, the cryostat housing a magnet which, in use, provides a substantially homogeneous magnetic field in the imaging region, further comprising a refrigerator arranged to cool certain elements of the cryostat, said refrigerator comprising a magnetic material which, in use, oscillates along an axis of the refrigerator. The cryostat is arranged such that the axis of the refrigerator is substantially tangential to a circle centred on the axis of the cylindrical cryostat.

The present invention relates to a cryogenic magnet assembly. More particularly it relates to a particularly advantageous arrangement of the refrigerator with respect to the remainder of the system.

MRI magnet systems are used for medical diagnosis. A requirement of an MRI magnet is a stable, homogeneous, magnetic field. Typically, cryogenically cooled superconducting magnets are employed. In order to achieve stability it is common to use a superconducting magnet system which operates at very low temperature, the temperature being maintained by cooling the superconductor, typically by immersion, with a low temperature cryogenic fluid, typically liquid helium. Cryogenic fluids, and particularly helium, are expensive fluids, and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogenic liquid used.

The present invention particularly relates to the structure and placement of a refrigerator interface. The interface serves to connect a refrigerator to a cryogenically cooled superconducting magnet in order to refrigerate one or more thermal shields, or to a cryogen vessel, or both, whilst at the same time ensuring that the refrigerator can be more easily removed and replaced during servicing.

FIG. 1 illustrates a cryostat 10 suitable for housing a superconducting magnet for an MRI system, according to the prior art. The superconducting magnet system (not shown) typically comprises a set of superconductor windings for producing a magnetic field, a cryogenic fluid vessel 12 which contains the superconductor windings, one or more thermal shields 23 (not shown in FIG. 1) completely surrounding the cryogenic fluid vessel 12, and a vacuum jacket 14 completely enclosing the one or more thermal shields and the cryogenic fluid vessel 12. A homogeneous magnetic field is produced in an axial imaging region located in the bore 13 of the magnet.

It is common practice to use a refrigerator 16 to cool the thermal shields 23 to a low temperature in order to reduce the heat load onto the cryogenic fluid vessel 12, and thus the loss of cryogen, for example liquid helium (not shown in FIG. 1), by boil-off. It is also known to use a refrigerator 16 to directly refrigerate the cryogen vessel 12, thereby reducing or eliminating cryogen fluid consumption. In both cases it is necessary to achieve good thermal contact between the refrigerator and the object to be cooled. Achieving good thermal contact at low temperature is difficult, and whilst adequate thermal contact can be achieved using pressed contacts at the thermal shield, it becomes more difficult to achieve the desired thermal contact at the very low temperatures required to recondense the cryogen within the cryogen vessel 12. As refrigerator 16 needs to be removable for servicing, so the thermal contacts need to be removable, and the refrigerator must be able to be replaced with equally effective thermal contact.

Cryogen condensation provides a good means of thermal contact between a refrigerator and the cryogen to be cooled. Accordingly, if a cryogen vessel 12 is to be refrigerated the vessel cooling part 18 of the refrigerator 16 may be placed within the cryogen gas volume, as shown in FIG. 1. This means that the vessel cooling part 18 of the refrigerator is surrounded by the cryogen gas. Contact between the refrigerator cooling part 18 and the gas is very simple to make effectively. Such an arrangement however requires that the refrigerator must be kept nearly vertical so that convection currents in the cryogen gas do not conduct heat from higher temperature regions to lower temperature regions thus adding to the heat load on the cold parts of the system.

It is desirable to maintain a certain level of liquid cryogen in the cryogen vessel 12 to adequately cool the superconducting magnet and to provide an adequate reservoir of cryogen so that the magnet system can be transported to an operating site still containing liquid cryogen, so that the superconducting magnet remains at, or at least near, its operating temperature. Because condensation does not work if the cooling part of the refrigerator is below the surface of the liquid, it is desirable to mount the refrigerator so that the condensing part is as high as possible with respect to the cryogen vessel. An implementation of this sort of interfacing is shown schematically in FIG. 2.

FIG. 2 shows a refrigerator interfaced removably into a vertical access turret 20. European patent application EP 0260036 describes such an interface. The access turret 20 also provides access for services to the magnet system, including the magnet main current connections 22, which may be partly or wholly normal conductors or high temperature superconductors.

As shown in FIG. 2, a vacuum 26 is present between the cryogen vessel 12 and the vacuum jacket 14. A thermal shield 23 is present within the vacuum. The refrigerator 16 is placed within an interface 40. In the illustrated embodiment, the refrigerator is a two-stage refrigerator. The first stage of the refrigerator is housed within a first stage tube 42, while the second stage is housed within a second stage tube 48. The first stage cold end is thermally connected to the thermal shield 23 through the refrigerator interface by a connection flange 46. The second stage cold end 18 is exposed to the interior of the cryogen vessel 12.

Any magnetic material in the vicinity of the magnet will be magnetized by the field surrounding the magnet, and its magnetism will affect the homogeneity and magnitude of the imaging field B_(i) in the axial imaging region located in the bore 13 of the magnet. For magnetic materials which are stationary, any disturbance of the imaging field B_(i) can be compensated by a process known as shimming, in which extra fields are created in the imaging region which cancel the effect of the disturbing field. If there are moving magnetic materials in the vicinity of the magnet, shimming cannot compensate, and the imaging field B_(i) is disturbed, resulting in degradation of the MRI image.

It is evidently desirable to reduce such time-varying interferences to a minimum. Conventional means for shielding magnet systems such as that shown in FIG. 1 include the following arrangements. A Faraday cage around the magnet system 10 can shield it from high frequency interference originating outside the cage. A magnetically soft steel cage will reduce the effects of low frequency magnetic interference originating outside the cage.

Certain types of refrigerator 16 include magnetic materials, such as regenerator materials, which oscillate during operation of the refrigerator. In known systems, this oscillation is along an axis 24 of the refrigerator, in a direction radial to the cylindrical cryostat, as illustrated in FIG. 1. As these refrigerators are used to cool the MRI system, they are in close proximity to the magnet, and are usually situated on or partially inside the vacuum jacket 14 of the cryostat 10, and therefore cannot be shielded by the conventional means mentioned in the preceding paragraph. The present invention aims to reduce the deleterious effect of such refrigerators on the homogeneity of the magnetic imaging field B_(i). Field contour lines 19 in FIG. 1 represent surfaces of equal magnetic field strength. As can be seen in FIG. 1, motion of the magnetic material along axis 24 will cut across many surfaces of equal magnetic field strength.

The refrigerator 16 is a mechanical device and such is subject to wear. The refrigerator must be serviced at regular intervals and must be replaced after a certain time in order to maintain adequate performance. The refrigerator can weigh up to 20 kg, and must be lifted out of the interface turret 20. A standard refrigerator interface according to the prior art fits the refrigerator towards the top of the magnet system, as illustrated in FIG. 1. This means that the refrigerator is awkward to remove for service as the engineer must reach over the body of the cryostat in order to remove the refrigerator from its interface. MRI magnet operating sites frequently do not have much clearance between the top of the system and the ceiling of the room housing the system, so it is frequently awkward to remove and replace the refrigerator safely. It is desirable that this operation can be carried out by one person and without the use of tooling. It is desirable therefore that the position of the interface be such as to facilitate service operations.

MRI magnet systems use refrigerators 16 to reduce the heat load onto the cryogen vessel 12 in order to reduce or eliminate the consumption of cryogenic liquid such as liquid helium. The refrigerators must be de-mountable from the magnet system to enable servicing and replacement, and are typically inserted into a sock, also known as a sleeve, 20 which interfaces the refrigerator in a heat conducting manner to the magnet system. Current practice puts the refrigerator interface 20 towards the top of the system 10 and with the axis 24 of the refrigerator pointing radially towards the axis of the cylindrical magnet system.

U.S. Pat. No. 5,782,095 describes a cryogen recondensing superconducting magnet system in which the refrigerator is placed almost horizontally, in order to minimise magnetic interference. The axis 24 of the refrigerator is substantially parallel to the axis of the cylindrical magnet. However, it has been found that most cryogenic refrigerators operate most effectively in a vertical or near-vertical orientation.

The present invention addresses at least some of the drawbacks of the known systems, and accordingly provides apparatus and/or methods as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments of the invention, given by way of examples only, in conjunction with the accompanying drawings wherein:

FIG. 1 shows a cryostat including a refrigerator housed in an interface sleeve, according to the prior art;

FIG. 2 shows more detail of the refrigerator and its interface as shown in FIG. 1;

FIG. 3 shows a cryostat similar to that of FIG. 1, modified according to an embodiment of the present invention;

FIG. 4 shows detail of the refrigerator and its interface sleeve, as illustrated in FIG. 3.

FIG. 5 shows detail of the mechanical assembly of an upper part of the refrigerator into its sleeve;

FIG. 6 shows detail of the mechanical assembly of a lower part of the refrigerator into its sleeve; and

FIG. 7 shows alternative detail of the mechanical assembly of a lower part of the refrigerator into its sleeve according to a further embodiment of the present invention;

The prior art provides a refrigerator interface which retains the refrigerator such that its axis is pointing in a vertical direction radial to the cylindrical magnet system. The magnetic materials of the refrigerator therefore move in a radial direction 24, crossing a number of magnetic field lines 19 and so have a large disturbing effect on the imaging field B_(i).

According to an aspect of the present invention, as illustrated in FIG. 3, the refrigerator interface is moved to the side of the magnet system 30. In a preferred embodiment, the refrigerator 16 is tilted slightly from the vertical, for example within 20° of vertical, so that the axis 24 and so the movement of the magnetic parts is substantially tangential to a circle 32 perpendicular to, and centred on, the axis A of the cylindrical cryostat 30.

The placement of the refrigerator and its access turret 34 at the side of the magnet system as shown in FIG. 3 assists with the removal of refrigerator 16 for servicing, as engineers no longer have to reach over, or even climb on to, the top of the system within a usually restricted access space.

Furthermore, the placement of the refrigerator and its access turret 34 at the side of the magnet system as shown in FIG. 3 moves the moving magnetic parts further from the imaging field B_(i), thereby reducing their effect on the homogeneity of the imaging field B_(i), and thus also the final image. The substantially tangential direction of movement of magnetic material along the refrigerator axis 24 means that the interference of the magnetic moving parts is further reduced since the distance of the moving magnetic material from the imaging field B_(i) is now nearly constant for all operation positions of the refrigerator. The moving magnetic material will not cross many magnetic field lines 19, and interference will be significantly reduced.

FIG. 3 shows a schematic of the positioning of the refrigerator 16 and the interface 40. The details of the interface resemble those shown in FIG. 2. The upper end of the interface is attached to the outer vacuum jacket 14. In a preferred embodiment, a helium transfer pipe 36 connects a condensation chamber at the bottom of the interface to the interior of the cryogen vessel 12. It is clear from FIG. 3 that the axis 24 of movement of the magnetic refrigerator parts is substantially tangential to a circle 32 centred on the axis A of the cylindrical cryostat 30. The interface can be tilted a small amount, for example within 20° of vertical, to make the axis 24 of movement of the magnetic parts more tangential. While the axis of the refrigerator may be inclined to a true tangential arientation, it is preferred that such inclination should be of an angle of less than 20°.

According to an aspect of the present invention, the refrigerator should be placed such that the axis of the refrigerator is substantially tangential, within the length of the refrigerator, to a circle centred on the axis A of the cylindrical cryostat. The axis of the refrigerator is substantially in the plane of the circle, and the axis of the refrigerator is located in a plane approximately midway along the axis of the cylindrical cryostat. The benefits of this arrangement are as follows. Cylindrical cryostats housing magnets such as described in the present invention typically comprise shielding coils near the ends of the cryostat, closer to the outer surface of the cryostat than the main field magnet coils. Near these shield coils, the local magnetic field is relatively high, and the field gradient is relatively steep. By locating the refrigerator according to the invention in a plane approximately midway along the axis of the cylindrical cryostat, the refrigerator is placed symmetrically with respect to the cryostat, and as far away from the shield coils as is possible. By so placing the refrigerator in the region of least field, and shallowest field gradient, any remaining interference of the refrigerator in the imaging process will be minimised. Furthermore, by locating the refrigerator in the region of lowest possible field, there is a reduced attractive or repulsive force on the components of the refrigerator. This in turn leads to reduced wear and more efficient operation of the refrigerator when placed as defined by the present invention. Preferably, the refrigerator is arranged with its axis near vertical, or at least within 20° of vertical.

Such orientation of the refrigerator makes for efficient operation of the refrigerator, and avoids wear of the moving parts of the refrigerator. While it is known to place refrigerators at other angles, even substantially horizontally, these orientations reduce the efficiency of the refrigerator and increase wear. The preferred orientation of the refrigerator, in a near-vertical position approximately midway along the cylindrical cryostat provides particularly good access for servicing and replacement of the refrigerator.

FIG. 4 shows the interface 40 in greater detail. The upper thin walled tube 42 of the interface connects between the room temperature flange 44 and the first stage connection flange 46. The lower thin walled tube 48 with its base 50 provides an enclosure for the second stage of the refrigerator, with adequate space around the recondenser 52 for gas circulation so that recondensation can occur effectively. The pipe 36 which connects 50 to the helium vessel is fitted so that its lower edge is below the base of the recondenser, and is angled downwards so that liquid will run into the helium vessel. The pipe 36 is sufficiently long and is preferably flexible, in certain embodiments being partly or wholly convoluted to increase flexibility, to accommodate movement between the interface 40 and the cryogen vessel 12 during transportation. The tube 36 is large enough to permit liquid to flow into the cryogen vessel 12 and to simultaneously allow vapour to counterflow into the recondenser. The room temperature flange 44, upper 42 and lower 48 tubes, the connection flange 46, the base 50, and the tube 36 with its connection to the cryogen vessel 12 are joined to make a leak tight assembly 40 with respect to the vacuum enclosed between the outer vacuum jacket 14 and the cryogen vessel shell 12. The joining may be done by brazing or welding as appropriate.

The thin walled tubes 42 and 48 are preferably made of stainless steel so as to have low thermal conductivity. The first stage connection flange 46 is preferably made of high conductivity copper to have good thermal conductivity. The pipe 36 is preferably made of stainless steel for ease of connection to the helium vessel. Other materials which have the desirable properties may be used for these components.

The thin walled tubes 42 and 48 are preferably made to be a close fit to the outer upper and lower tubes of the refrigerator 16 so as to reduce the amount of heat conducted to the first stage and the recondenser through the gas enclosed between the tubes and the refrigerator and through the tubes themselves. It is possible that the lower part of thin walled tube 48 might be expanded to make greater clearance to the recondenser for more favourable gas circulation for recondensation.

The connection flange 46 is connected to the thermal shield 23 by a flexible link 54 which is thermally connected to the connection flange 46 and to the shield 23 by bolting or welding or other good thermal connection as appropriate. Thermal shield 23 a is also connected to the flange 46 so as to shield the low temperature region of the interface 40 from high temperature radiation.

The condenser 52 is preferably grooved so as to increase its surface area for condensation. The grooves preferably run in a vertical direction so that the condensation flow is enhanced.

A high current electrical link 56 connects from the cryogen vessel 12 to the base 50 of the interface. In the access turret configuration used on certain magnet systems (for example as described in United Kingdom patent publication number GB2386676), the magnet current return path may be through the cryostat, and in this case current could flow in the flexible tube 36, creating undesirable heating. The electrical link 56 is provided to provide an alternative low resistance electrical path at this point to prevent heating of the tube 36.

FIG. 5 is a more detailed view of the sealing of the refrigerator to the interface and the connection to the connection flange 46. Room temperature flange 44 contains a ring 21. Top flange 28 is connected in a vacuum-tight manner to outer vacuum jacket 14, and contains a step 60 which fils closely to ring 21. An “O”-ring 64 is compressed against the smoothly machined faces of flange 58 and ring 21 by ring 66 and makes a leak-tight seal. The ring 66 is tightened against flange 58 by means of a number of screws 68. The refrigerator 16 is held into the interface 40 by a number of screws 70 which may have means of tensioning such as disc springs 72.

The connection between flange 28 and outer vacuum jacket 14 might also comprise a means of vibration isolation between the refrigerator interface and the room temperature flange 44 as revealed in patent EP 0260036.

The thermal connection between the refrigerator and the connection flange 46 may be made in a number of ways, one of which is shown in FIG. 5. A taper element 74 of high conductivity material, typically copper, is thermally attached to the first thermal stage of the refrigerator 76 and makes thermal contact with a mating taper in the connection flange 46, the contact being made by axial force exerted on the taper contacts. The angle of the taper is chosen to give high pressure between the contacting faces in order to achieve good thermal contact, but is not so shallow that the two tapers lock together and the refrigerator cannot be removed for servicing. Other means of making removable thermal connection are possible, in particular a pressed contact between flat faces with an indium metal washer as disclosed in patent EP 0260036, or a pressed contact between flat or tapered surfaces augmented by suitable grease between the surfaces, or augmentation of the pressed surfaces by other soft material which deforms to provide good thermal contact.

FIG. 6 shows a view of the recondenser end of the interface with an expansion 78 in diameter of thin walled tube 48, to facilitate gas flow for recondensation. The tight fit of the upper part of thin walled tube 48 reduces the heat conduction through the gas 80 contained between the tube 48 and the refrigerator second stage tube 82. The decreased diameter of the tube reduces the cross section of tube to conduct heat to the recondenser. Nevertheless, the inner diameter of tube 48 must be large enough to permit passage of recondenser 52 so that refrigerator 16 can be removed for servicing.

FIG. 7 shows another variant on the means of connecting the refrigerator recondenser 52 to the cryogen vessel 12. In this variant, the recondenser 52 sits within the cryogen vessel 12. A short vertical tube 84 is set into the side of the cryogen vessel 12. In this embodiment, the thin walled tube 48 includes a flexible section 86 to accommodate possible relative movement of the cryogen vessel 12 with respect to the vacuum jacket 14.

Refrigerators used for very low temperature refrigeration contain magnetic materials. The movement of these magnetic materials during operation of the refrigerator degrades the MRI image. Moving the refrigerator interface to the side of the system whilst retaining its height permits the refrigerator to be further from the axial imaging region formed in the bore of the magnet and also the axis of the refrigerator to be more tangential so that the interference to the MRI image is decreased, and permits easier servicing of the refrigerator. 

1. A substantially cylindrical cryostat defining an axial imaging regions the cryostat housing a magnet which, in use, provides a substantially homogeneous magnetic field in the imaging region, further comprising a refrigerator arranged to cool certain elements of the cryostat, said refrigerator comprising a magnetic material which, in use, oscillates along an axis of the refrigerator, wherein the axis of the refrigerator is substantially tangential, within the length of the refrigerator, to a circle perpendicular to, and centred on, the axis of the cylindrical cryostat, characterised in that the circle is located approximately midway along the axis of the cylindrical cryostat, and the refrigerator is placed at the side of the cryostat.
 2. A cryostat according to claim 1 wherein the axis of the refrigerator is substantially in the plane of the circle.
 3. A cryostat according to claim 1, wherein the axis of the refrigerator is within 20° of vertical.
 4. A cryostat according to claim 3 wherein the axis of the refrigerator is substantially vertical.
 5. (canceled) 